The dimorphic fungus, Histoplasma capsulatum (Hc) is found world-wide, and endemic to the Midwestern and southeastern United States. The organism can cause a life-threatening infection in immunocompetent or immunosuppressed individuals. One of the major risk factors for acquisition of disseminated Hc is the use of the TNF-? antagonist. In the past 3+ years, we have examined the role of the chemokine receptor, CCR5, in hastening the clearance of Hc. Our data demonstrate that this receptor is pivotal in dictating the balance between regulatory T cells and Th17 cells. In the CCR5 knockout mice or in mice that lack one of this receptor's major ligand, CCL4, the number of regulatory T cells in lungs is diminished whereas the number of Th17 cells is elevated. Neutralization of IL-17 reverses the salutary effect of a lack of CCR5 signaling. As an extension of these studies, we examined if the inimical effects of anti-TNF-? on host defenses to Hc were mitigated in the CCR5 knockout mice. We pursued this consideration since regulatory T cells arise after anti- TNF-? treatment and are responsible, in part, for the demise of animals. The absence of CCR5 mitigated the deleterious consequence of anti-TNF-? treatment. This finding has rekindled an interest in understanding the cells that drive the expansion of regulatory T cells in Hc-infected mice given anti-TNF-? and in examining how the lack of this cytokine or regulatory T cells blunt the innate immune response. Our preliminary data demonstrate that anti-TNF-? has a profound effect on recruitment of inflammatory monocytes and dendritic cells to the infected lung. Moreover, the cells that do reach the lungs are grossly impaired in their ability to inhibit the growth of Hc. In this proposal we will pursue aims to enhance knowledge regarding how anti-TNF-? promotes Treg expansion and how the lack of this cytokine and/or regulatory T cells alter recruitment and function of mononuclear phagocytes. Specific aim 1 will endeavor to identify a population or populations of dendritic cells or monocyte/macrophages that promote expansion of regulatory T cells in the absence of TNF-? and to investigate the phenotype of regulatory T cells that arises in TNF-?-neutralized mice. Aim 2 will examine why recruitment of inflammatory monocytes and dendritic cells is defective. We will attempt to separate the contribution of the lack of this cytokine from that of regulatory T cells. Aim 3 will pursue how antibody to TNF-? or regulatory T cells cause dysfunction of monocyte/macrophages and dendritic cells. These studies will generate a better understanding of the mechanisms by which anti-TNF-? enhances susceptibility to infection.